The significance of a fast and precise control of valve systems is increasing with increasing demands made of hydraulic systems. One example of such a field of activity is fuel injection, for example the direct injection of diesel fuel into the combustion chamber of a motor. What is referred to as the "common rail" system thereby has great potential, whereby the fuel from a central conveying pump is conveyed in a fluid conduit ("common rail") shared by all cylinders. The dosing of the fuel ensures via a system for fuel injection individually allocated to each cylinder. The improvement in motor operating behavior that can be achieved with the assistance of a common rail injection system thereby essentially results from an injection pressure up to 2,500 bar that can be controlled independently of the motor speed. Added thereto given this technology is the possibility of shaping the injection curve, i.e., generating a single or multiple pilot injection or the control of the injection rate, as well as of the free characteristics control of injection start and injection quantity.
For realizing these advantages, the system for fuel injection must meet an extremely high dynamic demand, for example it must exhibit a short operating dead time and a short switching time. Up to now, the control of common rail injectors has essentially occurred with the assistance of a solenoid drive. In individual cases, the injector is also controlled with the assistance of a piezo-hydraulic drive.
In the control of a fuel injector with the assistance of a piezoelectric direct drive for valve control of the hydraulic system, the problem arises, for example that only an inadequate compensation of a length modification of piezo actuator and housing caused by temperature or by aging and settling effects can be realized.
Added thereto is that a piezo actuator having a great structural length is required given the piezo direct drive, this being disadvantageous in terms of fabrication technology and in view of the manufacturing costs. Given a combination of the piezo actuator with a membrane hydraulics for valve control in the injection system, various problems occur such as, for example, a complicated mechanical balancing, a risk of rupture of the membranes as well as a low efficiency of the membrane hydraulics. Further unsatisfactory are, for example, the influence of pressure waves, a problematical temperature compensation as well as what is only an unsatisfactory switching behavior.
A further example of the use of a fast valve control is the braking circulation of a vehicle, whereby the hydraulic pressure in an anti-blocking system must be quickly and precisely regulated by the motor electronics. The use of a fast and precise valve control in the hydraulic circulation for controlling an elevator control or, respectively, vertical rudder in aircraft is also conceivable. Particularly given modem aircraft that are designed aerodynamically unstable, the guidance rudder must be driven very fast, so that the security of the aircraft is assured by an electronic stabilization of the flight attitude.
Therefore, there is a need for precise valve control that also reduces the affect of an influence on the switching behavior caused by operations or aging.